region this sinus presents the appearance of a cleft, the outer wall of which consists of the firm tissue of the sclera, while its inner wall is formed by a triangular mass of trabecular tissue (Fig. 870); the apex of the mass is directed forward and is continuous with the posterior elastic lamina of the cornea. The sinus is lined by endothelium and communicates externally with the anterior ciliary veins.

The aqueous humor drains into the scleral sinuses by passage through the pectinate villi which are analogous in structure and function to the arachnoid villi of the cerebral meninges.1

Structure.The sclera is formed of white fibrous tissue intermixed with fine elastic fibers; flattened connective-tissue corpuscles, some of which are pigmented, are contained in cell spaces between the fibers. The fibers are aggregated into bundles, which are arranged chiefly in a longitudinal direction. Its vessels are not numerous, the capillaries being of small size, uniting at long and wide intervals. Its nerves are derived from the ciliary nerves, but their exact mode of ending is not known.

The Cornea.The cornea is the projecting transparent part of the external tunic, and forms the anterior sixth of the surface of the bulb. It is almost circular in outline, occasionally a little broader in the transverse than in the vertical direction. It is convex anteriorly and projects like a dome in front of the sclera. Its degree of curvature varies in different individuals, and in the same individual at different periods of life, being more pronounced in youth than in advanced life. The cornea is dense and of uniform thickness throughout; its posterior surface is perfectly circular in outline, and exceeds the anterior surface slightly in diameter. Immediately in front of the sclero-corneal junction the cornea bulges inward as a thickened rim, and behind this there is a distinct furrow between the attachment of the iris and the sclero-corneal junction. This furrow has been named by Arthur Thomson2